1. Field of Invention
The present invention relates to the aluminum forming and heat treating arts, and more particularly to the aspects of those arts touching upon the production of intricate aluminum alloy parts which must be tough, very hard, and highly resistant to corrosion under stress. Even more specifically the invention deals with aluminum alloy sheets which are processed to become exterior panels of aircraft, spacecraft and shuttlecraft.
2. Prior Art
Since the discovery in the 1910's that most aluminum alloys can be heat treated to vary their physical characteristics, aluminum metallurgists and manufacturers have developed and now make routine use of a widely recognized scale of "tempers", ranging from "O" and "W" in the softest condition to "T8" in the hardest condition, (as used herein, T8 may also mean and includes T81, T851, T8511, etc., and similarly "T3" may also mean and includes T351, T3511, etc). These tempers not only quantify the hardness of the product, but other characteristics as well -- for example yield strength, elongation or ductility and stress corrosion resistance. In addition, they generally indicate the previous metallurgical treatment of the aluminum part, and thus give the reader a good idea of the microstructure of the material, such things as grain size, amount of precipitation of intermetallic compounds at the grain boundaries, and the extent to which such compounds have coalesced. Temper designations have become so important that no description of an aluminum alloy part is regarded as complete unless the temper is specified. Thus 2024-O aluminum sheet is one in which known percentages of alloying elements are interspersed in the aluminum matrix and the material has maximum softness and ductility, whereas 2024-T81 aluminum sheet has exactly the same chemical composition but has been heat treated to a very hard condition, making it much more suitable for many applications but at the same time making it much more difficult to form into the desired shape.
The predicament of the present inventor was that he was required to provide a compoundly curved part with a T851 temper (as illustrated in the accompanying drawing), and there was no known procedure for making such parts. It had been customary to form sheets with the as-delivered T351 mill temper to the desired shape and then heat treat them to the hard T851 temper, but in this instance the final form required was so intricate it could not be fabricated from material with a T351 temper.
On the other hand, it was not possible to obtain the desired result by forming the sheets in the "O" condition to the desired shape and then treating them directly to a T851 temper. The maximum temper possible in such procedure is T62, which is closer to the goal but lacks the properties required of the aluminum with a T851 temper.
Neither the prior patented art nor the published technical literature suggested a solution to this problem.